1) Field of the Invention
The present invention relates to an optical active device using a two-dimensional slab photonic crystal, in which an optical control function is improved, in an optical device used for optical transmission and optical information processing.
2) Description of the Related Art
A photonic crystal is an optical material having a periodic refractive index distribution, and has a band structure, with respect to an optical energy, that is the same as for a solid crystal, including a band structure, with respect to an electronic energy due to a periodic potential distribution. A photonic crystal is obtained by, for example, periodically arranging materials having different refractive indexes multidimensionally. Such a photonic crystal reveals new optical properties such as dispersibility and anisotropy that cannot be obtained by conventional optical materials, and hence has been noted as a next generation element in optical communication and optical information processing.
As an optical device using such a photonic crystal, there is a two-dimensional slab photonic crystal optical device. When this two-dimensional slab photonic crystal optical device has a sheet form, it operates as a photonic crystal optical device showing optical properties based on the photonic band theory, by realizing confinement of light like a thin material layer sandwiched between upper and lower air layers, in a direction perpendicular to the sheet face, and forming a two-dimensional periodic structure in a direction parallel with the sheet face. This conventionally proposed two-dimensional slab photonic crystal optical device has a linearly defective waveguide in the two-dimensional photonic crystalline structure described above, and a point (isolated) defect is added near this linearly defective waveguide. By having such a configuration, an optical waveguide device can be obtained, which has an ADD/DROP filtering function capable of adding to or taking from beams having a specific wavelength relative the linearly defective waveguide (for example, see document 1).
Such a conventional two-dimensional slab photonic crystal optical device, however, comprises a uniform material or an inactive material, and does not have an active layer as an active constituent, and hence it only functions as a passive device. Therefore, various two-dimensional slab photonic crystal optical devices having an active layer as an active constituent have been proposed (for example, see documents 5 and 6).
In the document 5 is disclosed an optical device having a slab optical waveguide on the surface of a substrate, and also having photonic crystals formed by regularly arranging in a lattice form a refractive index changing region, having a refractive index different from that of a core layer of the slab optical waveguide, in a part of the slab optical waveguide, which has a photonic crystal waveguide portion comprising, a single or annular optical active region formed in a depletion region surrounded by photonic crystals where a refractive index changing region is not provided, an optical waveguide region connected to the optical active region and formed so as to cross the photonic crystals, and an excitation unit with which the optical active region is excited. In this optical device, stimulated emission is caused by using Bragg diffraction of light generated in the active layer by a refractive index periodic structure, to thereby generate laser oscillation.
The document 6 discloses a two-dimensional semiconductor optical crystal element, in which a dielectric layer having a low refractive index is plane-contacted with at least one surface of the semiconductor optical crystals having a two-dimensional periodic structure that has a periodically vertical hole structure.
Various photonic crystal lasers are proposed as the optical active device that has an active layer as an active constituent, and uses a defect mode based on a two-dimensional photonic band structure (for example, see documents 2 to 4, and document 7). In the document 2 is disclosed a photonic crystal laser constructed by fusing a wafer formed by sequentially laminating a normal n-type cladding layer, an active layer and a p-type cladding layer, on a wafer in which an n-type cladding layer having a triangular lattice-form two-dimensional periodic structure is formed on an n-type substrate. The n-type cladding layer having the two-dimensional periodic structure forms a refractive index periodic structure by uniformly forming a periodic structure using air holes on a two-dimensional plane, and is such that defects are not introduced in this periodic structure. The principle of oscillation by the photonic crystal laser is such that oscillation occurs in a portion where the group velocity of the photonic band structure becomes 0, that is, at a band end in the photonic crystal mode (hereinafter “photonic crystal slab mode due to two dimensions”).
In the document 3 is disclosed a photonic crystal laser using the whole refractive index periodic structure on a two-dimensional plane is used as a laser oscillator, in photonic crystals having a structure in which an active region and a two-dimensional refractive index periodic structure are overlapped on each other, in order to strengthen the photonic crystal effect. A photonic crystal laser having a similar structure to that in this document 3 is disclosed in the document 5. The document 7 discloses a semiconductor laser comprising an active layer comprising a semiconductor and having optical amplification, and a phase shift section in which the luminous energy is coupled to only an optical mode in a predetermined direction, in a photonic crystal having a refractive index periodic structure on the order of the wavelength of light, as a semiconductor laser that can reduce a loss of injected energy and can obtain a high optical output by small electric power. Further, the document 4 discloses a photonic crystal laser in which a point defect is introduced in one point in a two-dimensional refractive index periodic structure to thereby use this point-defect region as a resonator.    Document 1: A. Chutinan, M. Mochizuki, M. Imada, and S. Noda, “Surface-emitting channel drop filters using single defects in two-dimensional photonic crystal slabs”, “Applied Physics Letter”, 2001, 79, 2690–2692.    Document 2: Masahiro Imada, Susumu Noda, Alongkarn Chutinan, Michio Murata, and Goro Sasaki, “Semiconductor Lasers with One- and Two-Dimensional Air/Semiconductor Gratings Embedded by Wafer Fusion Techinique, “IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS”, VLO. 5, No. 3, MYA/JUNE 1999, p. 658–663.    Document 3: Han-Youl Ryu, Jeong-Ki Hwang, Young-Jae Lee, and Yong-Hee Lee, “Enhancement of Light Extraction From Two-Dimensional Photonic Crystal Slab Structures”, “IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS”, VLO. 8, NO. 2, MARCH/APRIL 2002, p. 231–237.    Document 4: O. Painter, J. Vuckovic, and A. Scherer, “Defect modes of a two-dimensional photonic crystal in an optically thin dielectric slab”, “Journal of Optical Society of America B”, Vol. 16, No. 2/February 1999, p, 275–285.    Document 5: Japanese Patent Application Laid-Open No. 11-330619 (p. 2, p. 6, FIGS. 1 to 3).    Document 6: Japanese Patent Application Laid-Open No. 2000-232258 (p. 2 to 4).    Document 7: Japanese Patent Application Laid-Open No. 9-64458 (p. 2, FIG. 1).
Normally, in the photonic crystal laser, the mode is determined by the refractive index periodic structure. However, in the photonic crystal laser shown in the document 2, there is a problem in that there is no control unit that oscillates the laser beam in a desired mode, because freedom in controlling the mode is small, and the band structure and the band ends are dense, thereby a specific mode selection between respective modes is difficult. Further, the photonic crystal effect is obtained by having a structure in which the active layer and the cladding layer having a two-dimensional refractive index periodic structure using air holes are separated vertically with respect to the layered plane, and using leakage of optical electric field from the active layer. In such a structure, however, there is a problem in that the photonic crystal effect becomes weak on the theory.
In the photonic crystal laser shown in the document 3, a laser resonator is constituted by the whole refractive index periodic structure uniformly formed in a two-dimensional plane, and a waveguide is not provided. Therefore, the light propagating in this resonator frequently crosses each boundary surface (in this case, air boundary) of the refractive index periodic structure permanently. Thereby, there are problems such that a loss due to imperfection of production precision in each boundary surface between the refractive index periodic structures becomes large, and that non-radiative recombination likely occurs on the boundary surface between the refractive index periodic structure and the active layer. The semiconductor laser shown in the document 7 also has a similar structure, such that the active layer is divided into sections by the refractive index periodic structures, and hence there are the same problems as in the document 3.
In the photonic crystal laser shown in the document 4, a point defect is introduced into the two-dimensional refractive index periodic structure, but since it is a point defect, there are problems such that the volume of the active layer is small, and it is difficult to obtain high output of the laser to be oscillated essentially and from the standpoint of cooling. Therefore, in this document 4, even if the size of the point defect that becomes the active region, is enlarged by filling the air holes constituting a plurality of adjacent refractive index periodic structures, in order to realize high output of the oscillation laser, a higher mode occurs, and a low-order single mode cannot be obtained. As a result, a high-quality resonator cannot be obtained. This photonic crystal laser realizes vertical confinement of laser beams with respect to the layered plane of the photonic crystal, by a photonic band gap due to the two-dimensional refractive index periodic structure, and fetch of the laser beams is carried out vertically of the upper and lower slabs. In other words, though the laser resonator performance for confining the laser beams and fetch of the laser output are theoretically contradicting structures, the photonic crystal laser tries to realize those two at the same position, that is, at the point defect, being the active region. As a result, there is a problem in that high-output laser cannot be obtained.
In the conventional techniques described above, only a laser is mentioned as the optical active device, and application to other optical active devices of photonic crystals has not been disclosed at all.